Description of a Wireless Local Area Network (LAN) Systems:
According to the wireless LAN standard (IEEE 802.11), a wireless LAN station needs to connect to an AP (assess point) for accessing a file server or other workstations of a network system. The AP connects distributed systems and a set of basic service areas. It provides the communication between a local station in a wireless LAN and a remote station in another wireless LAN. It also provides the capability of accessing distributed systems for a station.
FIG. 1 shows the typical hardware architecture of a wireless LAN system. As illustrated in FIG. 1, an Ethernet LAN 100 may have two basic service sets 101 (BSS1) and 102 (BSS2), one workstation 103 and one file server 104. Each basic service set comprises one access point and two wireless stations. Basic service set BSS1 includes the access point AP1 and two stations STA1 and STA2. Basic service set BSS2 includes the access point AP2 and two stations STA3 and STA4. Each wireless station should associate itself with one AP before it can communicate with the file server 104 or the workstation 103 in the Ethernet LAN 100.
When a wireless LAN station is powered on, it first looks for an AP. After it finds an AP, the wireless LAN station registers itself with the AP. The station can then synchronize with the AP and, thereafter, transmit and receive information packet to and from the AP.
According to the wireless LAN standard FHSS (Frequency Hopping Spread Spectrum) (IEEE 802.11), the bandwidth used in the RF (radio frequency) is between 2.40 Ghz and 2.50 Ghz among 79 channels in the US and Canada areas. The frequency hopping radio signal shall hop in the frequency band between 2.40 Ghz and 2.50 Ghz as shown in FIG. 2. The AP shall be the timing master and shall perform the TSF (timing synchronization function) to keep the timers for all wireless stations in the same BSS synchronized. For each channel, the AP shall periodically transmit special frames called beacons each containing a copy of its TSF timer and hopping sequence to synchronize other wireless stations in a BSS. The AP shall broadcast a generated beacon frame at a fixed time interval referred to as a beacon interval. A receiving wireless STA shall always accept the timing information in beacons sent from the AP servicing its BSS. If a wireless STA's TSF timer is different from the timestamp in the received beacon frame, the receiving wireless STA shall set the value of its local timer to the received timestamp value.
Traditionally, when an AP is powered on in a basic service set, it periodically broadcasts a beacon frame to all wireless STAs in the service set. The beacon frame contains the information for synchronizing with the AP and the information is referred to by the wireless STA looking for the AP. Because the beacon interval used in the traditional approach is long (100 mini seconds), it could increase the search time when wireless STA is looking for the AP.
A wireless LAN station needs to do a registration before it can communicate with an AP. According to 802.11 standard, the registration should include two phases. The first phase is authentication. This process is used to identify whether the station has the legal right or not to access the wireless network via a medium access control (MAC) layer. After the authentication is passed, the station needs to establish a connection link with the AP by sending an association request packet to the AP and waiting to receive a response frame from the AP which acknowledges the association.
The general steps taken by a traditional MAC sub-layer management architecture for a wireless STA from search to registration with an AP are shown in FIG. 3. According to the design of FIG. 3, a wireless STA shall perform the three phases: scanning phase, authentication phase and association phase.
In the scanning phase, all IEEE 802.11 STAs initially start from "IDLE" state (step 301). The radio hopping channel shall be set by the hardware (step 302). To search for the AP, a wireless STA shall listen in each channel and scan for any beacon frame transmitted by the AP (step 303). Upon finding the channel and receiving the AP beacon, the scanning phase is completed. Otherwise, the wireless STA shall listen in the next channel and return to step 302. A new radio hopping channel shall be set.
After receiving the AP beacon, the receiving wireless STA will join a BSS by setting its local hopping time and channel sequence according to the information contained in the AP beacon (step 304) in order to maintain synchronization with the AP. The remaining steps constitute the authentication phase and association phase.
In the authentication phase, IEEE 802.11 in general requires bi-directional authentication. At step 305, the wireless STA exchanges some packets with the AP for authentication. It may further include three steps of assertion of identity, challenge of assertion and response to challenge.
In the association phase (step 306), the wireless STA exchanges the association packets with the AP to accomplish the AP registration. The communication link between the wireless STA and the AP is thus setup. The wireless STA can start exchanging data frames with the AP (step 307).
Because the radio frequency hops between 2.40 Ghz and 2.50 Ghz, a wireless LAN station may take a long time to find an AP among the 79 channels in Federal Communications commissions (FCC) regulation.
Each frame used in authentication and association should be sent in complying with the rule of to 802.11 standard (Carrier Sense Multiple Access with collision Avoidance, CSMA/CA). The time consumed for a wireless station to complete all the processes of authentication and association may vary greatly, and is unpredictable, especially when the traffic in the wireless medium is busy. Therefore, the total time from searching for an AP to accomplish the registration is unpredictable for the wireless LAN stations that comply with 802.11 standard.
From the foregoing discussions of prior arts, it can be seen that the disadvantages as pointed out earlier make the conventional access scheme inappropriate for a wireless LAN station to connect with an AP. There exists a strong need for having a more efficient access scheme for the wireless LAN station to quickly access a wireless LAN through an AP.